The field of the invention is magnetic resonance imaging (MRI) and, in particular, local coils for use in MRI.
In MRI, a uniform magnetic field B0 is applied to an imaged object along the z-axis of a Cartesian coordinate system fixed with respect to the imaged object. The effect of the magnetic field B0 is to align the object's nuclear spins along the z-axis.
In response to a radio frequency (RF) excitation signal of the proper frequency oriented within the x-y plane, the nuclei precess about the z-axis at their Larmor frequencies according to the following equation:ω=γB0  (1)where ω is the Larmor frequency, and γ is the gyromagnetic ratio which is a constant and a property of the particular nuclei. The value of the gyromagnetic ratio γ or protons is 42.5759 MHz/Tesla.
In a typical imaging sequence for an axial slice, an RF excitation signal having a frequency centered at the Larmor frequency of the protons is applied to the imaged object at the same time as a magnetic field gradient Gz is applied. The gradient field Gz causes only the nuclei in a slice with a limited width through the object along an x-y plane to be excited into resonance.
After the excitation of the nuclei in this slice, magnetic field gradients are applied along the x- and y-axes. The gradient along the x-axis, Gx, causes the nuclei to process at different frequencies depending on their position along the x-axis, that is, Gx spatially encodes the precessing nuclei by frequency. The y-axis gradient, Gy, is incremented through a series of values and encodes the y position into the rate of change of phase of the precessing nuclei as a function of gradient amplitude, a process typically referred to as phase encoding.
A weak nuclear magnetic resonance generated by the precessing nuclei may be sensed by the RF coil and recorded as an NMR signal. From this NMR signal for a series of such signal acquisitions with different phase encodings, a slice image may be derived according to well-known reconstruction techniques. An overview of NMR image reconstruction is contained in the book “Magnetic Resonance Imaging, Principles and Applications” by D. N. Kean and M. A. Smith.
The RF excitation and the NMR signal may be transmitted and received, respectively, by means of one or more RF coils. Improvements in the signal-to-noise ratio of the received NMR signal can be obtained by placing “local coils” on the patient. The local coil having a smaller reception pattern can focus in on the region of interest to obtain a stronger signal and to receive less noise. A common local coil is a head coil providing for a generally cylindrical volume into which the patient's head may be placed.
During MRI, the acquired NMR signal can be thought of as filling a domain referred to as k-space. Typically, the NMR signal acquired for each different set of gradient fields completes one line of k-space data. While normally k-space data may be obtained on a row and column basis, it is also known to obtain k-space data in radial spokes through the center of k-space. After an area of k-space has been filled with data, a two-dimensional Fourier transform of the k-space data produces the MRI image.
The acquisition of each line of k-space data is preceded by the RF excitation signal described above. During this excitation, the MRI system may send the local coil a disable pulse allowing the local coil to “detune” itself electrically so as not to be overloaded by the RF excitation. This disable pulse may, for example, forward bias diodes that detune the coil.
The acquisition of the necessary k-space data is not instantaneous and requires that the patient remain substantially motionless during the acquisition period to avoid introducing artifacts into the reconstructed image. Regular physiological motion such as caused by the patient's breathing or heartbeat may be accommodated to some extent by careful selection of the gradient field order and timing. Non-periodic jerking or twitching motions, however, present a more difficult problem. Physical restraint of the patient may be impractical or distressing to the patient and only moderately successful.